1. Field of the Invention
The present invention relates to an organic photoreceptor used in the field of an image forming method, a manufacturing method of such an organic photoreceptor, and a process cartridge and an image forming apparatus using this organic photoreceptor.
2. Related Art
In recent years, an organic photoreceptor (hereinafter also referred to as “photoreceptor”) has been utilized over an extensive range in the electrophotographic photoreceptor. The organic photoreceptor has advantages over other types of photoreceptor, such as easier development of materials conforming to various types of light sources for exposure ranging from visible light to infrared light, possible selection of materials free of environmental pollution and lower production costs. However, the organic photoreceptor is characterized by poorer mechanical strength, easier deposition of foreign substances, poorer chemical resistance, earlier deterioration of electrostatic characteristics at the time of printing multiple sheets, and susceptibility to scratches on the surface.
To be more specific, an organic photoreceptor is required to have a sufficient resistance (wear resistance) to adhesion of foreign substances on the surface or damages caused by the external mechanical force applied when a toner image formed on the photoreceptor is transferred onto a transfer material such as paper or when toner remaining on the photoreceptor is removed by cleaning.
An art to employing polyarylate resin as a binder of the photoreceptor to improve the resistance of the photoreceptor or adhesion of foreign substances on the surface (Document 1, Japanese Patent O. P. I. 2003-195564).
Though a photoreceptor employing polyarylate resin improves anti-resistant characteristics, it is easy to generate cracks because of its rigidity, and toner particles or paper dusts are easy to adhere in the neighborhood of the damage to form filming once the damage is formed, and it causes easily insufficient image such as non-uniform image or dash marks (comet like small marks).
A method of impregnating the polyarylate resin of the photoreceptor with a fluorine-containing resin minute particles such as polytetrafluoroethylene (PTFE) is known in the prior art to improve the resistance of the photoreceptor to adhesion of foreign substances on the surface or damages so as to dissolve the problem of polyarylate resin (Document 2, Japanese Patent O. P. I. 2003-15326).
The fluorine-containing resin minute particles are apt to coagulate during the process of dispersing or coating of the coating composition when the fluorine-containing resin minute particles are incorporated in the polyarylate resin. Therefore, the surface energy of the photoreceptor is not sufficiently reduced and, as the result, prevent of adhesion of foreign material is not sufficient, dash-marks and cracks are apt to generated.
The art of using fine particles of fluorine-containing resin having a small crystallinity (the half-width of the X-ray diffraction peak: 0.28 or more) and a small diameter has been reported as a method of effectively reducing the friction coefficient on the surface of the photoreceptor to improve the wear resistance (For example, Document 3, Japanese Patent O. P. I. 8-328287).
However, the fluororesin particles tend to be coagulated and the stability of the dispersion is lowered so that the layer having uniform properties is difficultly formed since the spreading ability of the fluororesin fine particle having low crystallinity is larger in a coating dispersion using polyarylate as a binder. Therefore, the adhesion of foreign substances cannot be effectively prevented so that dash marks around the coagulated particles, unevenness of the image caused by scratches and burring of image tend to be caused. Moreover, when the coagulated particles of the fluororesin particles are increased in the surface layer, the coagulation interface of the resin particles tends to be a charge trap so that problems such as formation of a memory image such as transferring memory and increasing of the remaining potential accompanied with repeating use causing lowering in the image density tend to be caused.
On the other hand, recently, the image forming apparatus using electrophotographic system is frequently used as an information device for printing out an image formed by a computer. Accordingly, a copying machine and a printer capable of forming a high quality digital image are demanded. Therefore, a developing means capable of exactly developing the static latent image formed on the organic photoreceptor is required, and the use of a polymerized toner in which the distribution of the toner particles are made to monodispersed state is proposed as one of the above means (Document 4, Tokkai 2002-244336). However, some problems are found that the merit of the toner improved in the particle size distribution cannot be satisfactorily displayed when the static latent image is formed on the organic photoreceptor having roughened surface caused by the formation of the dash mark, unevenness and burring of image since the formation of the static latent image is disturbed and the roughened surface is more clearly reproduced as the toner image, even when the toner capable of exactly reproducing the static latent image is employed.
When fluororesin particles having an average particle diameter of not less than 0.20 μm are contained in the surface layer of the photoreceptor, the sharpness of image is lowered by scattering of the exposing laser light and the mechanical strength tends to be lowered and scratches are formed on the photoreceptor surface by degradation of the physical properties of the layer. Accordingly, a suitable photoreceptor cannot be always obtained.
An object of the invention is to simultaneously improve the anti-adhesion ability to foreign substance and the anti-scratch ability which are consistent from each other, and to provide an organic photoreceptor which has good electrical potential property for a long period and is capable of forming an electrophotographic image with high sharpness. In detail, the object is to provide an organic photoreceptor which has good electrical potential property for a long period and is capable of forming an electrophotographic image with high sharpness by preventing the occurrence of the dash marks, the image unevenness and the image burring by preventing the formation of the toner filming and cracks which tend to be caused by the use of the polyarylate resin binder having high resistivity to scratch on the surface layer of the photoreceptors, and to provide a processing cartridge and an image forming apparatus employing the photoreceptor.